Combines are very large pieces of equipment. The operator sits atop the machine in a high cab with full-length windows for a good view. A long, square body, topped with a grain tank and a chute through which the threshed grain can be removed from it, rides on huge, front-mounted drive wheels and smaller, skinnier rear steering wheels. A turbo-charged diesel engine propels the combine and provides power for the header, threshing cylinder, cleaning system, and augers that move the grain from the header through the threshing cylinder to the grain tank and then out to a waiting truck.
As a combine progresses along rows of grain, its front component-the reel, a large, hexagonal metal piece set parallel to the ground-rotates, sweeping the grain stalks up into the machine. Different reel designs are required to harvest different crops. For example, while a wheat reel shunts the stalks into a cutting bar that slices them just below the heads of ripe grain, a corn reel strips the ears of corn from the stalks, leaving them flattened against the ground. Today, farmers can choose from many different types and sizes of header models. The explanation below describes the progress of wheat through a combine.
Once inside the body of the combine, the stalks are thrust against the cutter bar, a component that looks something like a comb. With the "tines" of the comb catching the stalks, a knife-like implement slices them near ground level. The stalks are then transported up onto an elevator by means of a stalk auger. Large metal cylinders, augurs feature screw-like projections that trap the grain so that it can be transported. The stalk augur, set parallel to the ground, sweeps the cut stalks onto the elevator-a pair of rollers fitted with conveyor belts that carry the grain upwards into the heart of the combine, the threshing cylinder. This cylinder is a large roller with protuberances. Rotating at high speed over a slitted, half-moon-shaped trough (the concave), the cylinder separates the kernels of grain from the heads of the stalks.
Once separated from the kernels, the stalks are swept up by the bars of the threshing cylinder, which deposit them on the first of a series of straw walkers. These are large, slightly overlapping, square platforms that gradually descend toward the rear of the combine. Vibrating slightly, the first walker causes the straw to drop onto the second, and the second shakes until the straw drops onto the third and lowest, at which point it is either dropped through a chute onto the ground or, in a baling machine, packed into bales. Unlike the stalks of grain, the kernels are small enough to fall through the slits in the concave and are caught in the grain pan that lies beneath it. The grain pan vibrates, shaking the kernels, the chaff, and some heads that made it through the threshing cylinder intact into a set of vibrating sieves.
The vibrations cause the kernels to fall through the sieves, while the unthreshed heads are trapped. A fan adjacent to the sieves blows air across them, causing the chaff (which is very light) to blow backwards out the rear of the combine. The unthreshed heads are then routed, via another auger, into the tailings elevator, which transports them back into the threshing cylinder. Meanwhile, the kernels fall into the grain augur and are transported up into the grain elevator, which deposits them in the grain tank. Yet another augur, the unloading augur, is inserted into the grain tank, and grain can be removed from the tank through it.
